Chemical treatment of refractory metal surfaces



United States Patent CHEMICAL TREATMENT OF REFRACTORY METAL SURFACES George F. Otto, Oreland, Pa, assignor to Amchem Products, a corporation of Delaware No Drawing. Application November 20, 1%56 Serial No. 623,316

6 Claims. (Cl. 41-42) This invention relates to the pickling and brightening of certain refractory metals. More specifically, it has to do with the use of acidic pickling media in conjunction with oxidizing. agents in the treatment titanium and zirconium metals and their alloys.

Objects composed of zirconium or titanium, or of alloys in which these metals predominate, present unusual problems insofar as providing them with bright, clear, finished surfaces is concerned. This is because these metals, particularly at elevated temperatures, have a very strong afiinity for oxygen, hydrogen, nitrogen, and other scale-forming elements. Consequently, contact between these metals and gases of the type referred to, either during preliminary handling of the metals or in the final fabrication of them, often results in the formation of oxide films, or of surface scale, either of which will blemish the finished surface. Oxide and nitride scales are particularly troublesome, and the presence of such scale not infrequently renders the products unsuitable for the purposes for which they had been originally intended.

Surface scale and films of the type referred to adhere very tenaciously to both titanium and zirconium, and for thisreason, it has frequently been found that conventional pickling processes, making use of sulphuric acid or hydrochloric acid, for example, have been ineffective to remedy the difliculty. In order to pickle titanium and zirconium successfully, it has become customary to resort to the use of aqueous solutions containing a mixture of hydrofluoric acid and nitric acid, although the use of hydrofluoric acid alone has been proposed.

Generally speaking, the pickling baths most commonly used for treatment of these metals consist of a mixture of hydrofluoric acid and nitric acid in water, the hydrofluoric' acid representing froml% to 6% of the bath, by weight, and the nitric acid from 1 to 50%.

Hydrofluoric acid alone, in concentrations ranging up to 60% is quite capable of removing any scale or film likely to be encountered on the surface of titanium or zirconium objects, but it attacks the basis metal so actively that most metal finishers prefer to combine it with the slower-acting nitric acid.

But even when the acid content of the pickling bath consists mainly of nitric acid, with only a small proportion of hydrofluoric acid, the activity of the pickling solution is so great as to entail an extensive and costly loss of the basis metal. Furthermore, pickling these metals with such baths frequently leaves the surface in a seriously pitted condition.

An object of this invention is to provide an acid pickling bath which will satisfactorily remove surface contaminants from articles made of zirconium or titanium or alloys in which these metals predominate, without causing an objectionable loss of the basis metal. A concomitant object of the invention is the provision of pickling solutions which, in use, will cause little or no pitting of the pickled surface. Another object of the invention is the provision of treating solutions which are capable ting a very pronounced brightening of Zirconium ice and titanium surfaces to which they have been applied. Other objects and advantages of the invention will be apparent as this detailed description proceeds.

A fundamental concept of the present invention is the discovery that if one adds an oxidizing agent to a pickling bath for titanium and zirconium, which bath contains a mixture of nitric acid and hydrofluoric acid, the activity of the pickling bath with respect to surface contaimi nants, such as film and scale, is not substantially reduced, but there is nevertheless a significant lessening of the rate of attack of the acid on the basis metal, or, alternatively, or coordinately, a marked enhancement in the brightening effect of the bath upon the surfaces treated, and substantial elimination of surface pitting.

As oxidizing agents, I may use sodium salts of chlorine,

such as sodium chloride, sodium chlorate and sodium hypochlorite. Alternatively, I may use the corresponding chloride salts of potassium, or ammonium chloride. Still further, hydrogen peroxide may be used. Mixtures of compatible oxidizing agents are highly effective, the preferred composition being one which contains sodium chloride and sodium chlorate in equal proportions.

All of these compounds have the characteristic of serving as oxidizing agents for the basis metal, in the sense that they will cause the metal to manifest a higher valence.

Entirely apart from the problem of scale removal, the brightening of zirconium or titanium surfaces may be very effectively achieved by the combination of such oxidizing agents with the acids of the pickling bath. It has been discovered that, even at relatively low concentrations, the presence of such oxidizing agents in the composition uniformly results in the production of a more brilliant surface than is produced Without them, and often results in the production of a pickled surface having such a high degree of sheen as to completely eliminate the necessity of resorting to subsequent polishing operations.

As to the concentrations in which the oxidizing agents are used, it should first be pointed out that since one effect of these oxidizing agents is to inhibit the attack of the pickling acids on the basis metal, the presence of these agents tends to slow down the activity of the pickling bath. It is, therefore, necessary to increase the time allowed for the metal-treating operation as the proportion of oxidizing agent in the pickling bath is increased. Subject to this limitation, concentrations as low as 2 grams per liter, and as high as 50 grams per liter, fall within the limits of the commercially useful range. At concentrations below 2 grams per liter, the inhibiting effect is scarcely sufficient to be noticeable, except when the oxidizing material used is hydrogen peroxide, which gives good results at concentrations equivalent to as little as 1.5 grams per liter (weight/volume). Even at such low concentrations, however, a substantial brightening effect may be secured. At concentrations above 50 grams per liter, the process may be slowed down to such a degree as to appreach the point of diminishing returns. That is to say: the saving in basis metal. may be more than off-set by rising costs due to the increase in processing time required.

As a general rule, the amount of oxidizing agent present, in relation to the total weight of the solution, may desirably be varied in direct proportion to the amount of pickling acid which the bath contains. However, this is a factor which is strongly affected by time, temperature, the resistivity of the scale, and the degree of brightening desired. Under some circumstances, for example, it may be desirable to use a relatively low proportion of oxidizing agent even in a bath which contains a rather high concentration of picklingacid, and vice versa. Where the conditions under which an article was fabricated, for example, are such as to have resulted in the presence of relatively littlescale, and it is desired to perform the pickling and brightening operation at relatively high speed,

the acid concentration may be high and the oxidation agent concentration may be low. The reverse would be true where thin sections coated with refractory scale are to be treated. Here it is vital to keep the weight loss at a minimum, and yet remove the scale completely. Consequently, under these circumstances, the oxidizing agents should be used at higher levels of concentration than is normal, the acid, particularly the hydrofluoric acid, should be at low concentration, and the porcessing time should be increased.

It is easy to evaluate the effectiveness of the treatment in relation to the removal of surface scale and oxide film by simple visual examination, noting the brightness of the treated surface and the extent of surface pitting, if any.

In order to evaluate the effectiveness of the treatment in relation to reduction in Weight loss, the following testing procedure has been adopted as a standard: First, place several small test panels, known as coupons, of the metal to be treated in a pickling bath which contains nitric acid by weight and 2% hydrofluoric acid by weight of the total solution. Allow the coupons to remain in the bath for 30 seconds at room temperature, then remove them, wash with water, and dry. The purpose of this treatment is to ensurethat the surface of all coupons is uniformly and completely clean. After washing and drying, each coupon is individually weighed.

The next step in evaluating weight loss is to test a control. This is done by immersing one of thecoupons in a 20% nitric 2% hydrofluoric acid test bath at room temperaturefor exactly 60 seconds, and then rinsing, drying and re-weighing the control specimen. Weight loss attributable to the action of the pickling bath is measured in milligrams per square foot.

Another specimen, cleaned in the same way as the control, is immersed, after Weighing, in a pickling bath having exactly the same acid concentration, plus 2 grams per liter of the selected oxidizing agent. The coupon is permitted to remain in the bath for the same sixty-second interval at the same temperature, following which it is rinsed, dried and re-weighed.

The procedure is repeated, increasing each time the proportion of oxidizing agent present in the bath.

Table I, which follows, shows the results of three sets of tests conducted at different times, but here grouped together to afford a comparison of the results following use of ditferent oxidizing materials, namely, sodium chloride, sodium chlorate, and a mixture of equal parts of sodium chloride and sodium chlorate, in pickling baths applied to coupons of titanium metal.

TABLE I [Treatment of titanium metal for 60 seconds at room temperature in aqueous bath containing 20% HNO; and 2% HF] All coupons in any one column were cut from the same sheet, but the coupons in each series were cut from different sheets. This accounts for the difference in weight loss of the respectively difierent control coupons.

It Will be noted that with sodium chloride a concentration of 2 grams per liter is insufficient to produce any substantial reduction in weight loss. Indeed, at this concentration, there would seem to be some slight increase in metal loss, rather than decrease. But the F loss is fairly substantial even at 2 effect in brightening the surface is marked, even at this low concentration, as compared to the degree of brilliance produced by the same pickling bath in the absence of any oxidizing agent. When the concentration of sodium chloride is increased to 4 grams per liter, there is an appreciable reduction in the weight loss, and at 8 grams per liter, the reduction in weight loss is almost out in half, as compared to that incurred where no oxidizing agent is used. With the oxidizing agent at concentrations in the neighborhood of 15 g. p. 1., so high a degree of surface brilliance is secured in 60 seconds as to obviate the need for further polishing, and pitting of the surface does not occur.

When sodium chlorate is used, the reduction in weight grams per liter concentration, and at 50 grams per liter, the weight loss has been reduced by over However, it should be mentioned that the brilliance of the surface produced in an immersion time of only 60 seconds was very much less where the higher concentration of oxidizing agent was employed than it was where lower concentrations were used. In other words, the time required for attaining a desired degree of brilliance in the final surface would have to be very substantially increased in a pickling bath which contained 50 grams per liter of sodium chlorate.

This observation explains why it is preferable to use mixtures of oxidizing agents, as shown in the third column of Table I, where the oxidizing material was a mixture of equal parts of sodium chloride and sodium chlorate. When this mixture is used, the weight loss incurred in treating titanium is not reduced as significantly, at any given concentration, as when either sodium chloride or sodium chlorate is used alone. But for reasons not yet entirely clear, the effect of using a mixture of these two salts is to make it possible to produce the same degree of brightening of the final finish in a substantially shorter time, or, as related to Table I, to produce a much more brilliant sheen in sixty seconds than can be produced by using the pickling acids alone, or by using these acids with either sodium chloride or sodium chlorate alone.

Considering all of the factors involved, our preferred procedure in treating titanium is to use a pickling bath which contains 20% of nitric acid and 2% hydrofluoric acid by weight in relation to the total weight of the bath, and to add to this bath 15 grams per liter of. a mixture of equal parts of sodium chloride and sodium chlorate. With a bath of this composition, complete scale removal and the desired degree of surface brightening can be attained in sixty seconds at room temperature, with a reduction in weight loss in the neighborhood of 40%, in a majority of cases.

In conducting the tests reported in Table I above, titanium metal was employed. Parallel tests, conducted in exactly the same way, but with the substitution of zirconium coupons in lieu of titanium, show the following results:

TABLE II aqueous bath containing 20% HNOa and 2% HF Weight Loss in Milligrams per Sq. Ft.

Grams per liter used NaOl NaOlOs NaCl+ NaOlOa All of the tests reported in Table II were conducted at the same time, using coupons cut from the same sheet. Consequently, the weight loss in each control specimen is the same.

Where zirconium is treated, the optimum balance between brilliance of finish and reduction in weight loss occurs when the concentration of oxidizing agent is at a higher level than is preferred where titanium is being treated. It will be seen from an examination of Table II that the reduction in weight loss becomes substantial when the oxidizing agent is present in concentrations ranging upwards from 30 grams per liter, whereas, in the treatment of titanium, the preferred value is grams per liter.

It will also be noted that in treating zirconium, a mixture of sodium chloride and sodium chlorate affords a greater reduction in weight loss at the lower levels of concentration than is afforded if either of these oxidizing agents is used alone. This is not the case in treating titanium.

Although it is true that the presence of the oxidizing agent in the treatment of zirconium does not effect an equivalent reduction in weight loss until the concentration of oxidizing agent is relatively higher than in the treatment of titanium, the effect of the oxidizing agents in brightening of the surface is much more strongly marked at low concentrations when zirconium is treated than is observed in the treatment of titanium. Whereas, in the treatment of titanium, maximum brilliance is not secured in 60 seconds until the concentration of oxidizing agent is in the neighborhood of 15 grams per liter, it has been found that nearly maximum brightening took place even at the lowest levels of concentration where the oxidizing agent in a pickling bath applied to zirconium consisted of a mixture of sodium chloride and sodium chlorate in equal proportions.

Consequently, where the purpose is primarily to brighten the surface of the finished article, and the article is made of zirconium metal, the preferred oxidizing agent is a mixture of equal parts of sodium chloride and sodium chlorate at a concentration of 4 grams per liter. This is also the preferred concentration where scale removal, in addition to surface brightening, is required, since it permits operation of the bath at the fastest possible rate, and yet produces a completely satisfactory degree of surface finishing. The reduction in metal loss is relatively slightabout 12.5%, but the sheen is sufficiently well developed as to require no further polishing treatments whatever in most cases where this mixture is used in the treatment of zirconium.

If procedures analogous to those employed in arriving at the values given in Table I and Table II are applied to the evaluation of other oxidizing agents, results which may fairly be regarded as comparable will be secured.

For example, the weight loss on coupons of titanium metal in a pickling bath containing the same mixture of pickling acids as was used in the tests reported in Table I and Table II and containing varying concentrations of sodium hypochlorite showed values of the following order of magnitude:

When sodium hypochlorite is present as the sole oxidizing agent in the pickling bath, in quantities approximating 15 grams per liter, the weight loss is reduced by 27.5% as compared to the control, and at this value, a high luster is imparted to the surface in 60 seconds of treatment.

At a concentration of 50 g. p. 1., the weight loss, as compared to the control was reduced by 63%, but the production of optimum brilliance required treatment for a longer time. Even at only 4% concentration of oxidizing agent, weight loss was reduced by 14%, and here the degree of surface brightening produced in sixty seconds was excellent.

Hydrogen peroxide is an especially desirable oxidizing agent for use in this process, since as little as 0.15%

(weight/volume) has been found to be effective as the oxidizing agent in pickling baths of the type discussed. It is, however, more costly than either of the sodium salts just mentioned, and is more difficult to handle (especially from the standpoint of deterioration during storage). Furthermore, it is a little difficult, particularly for relatively unskilled labor, to make certain that precisely the desired proportion of active reagent is employed, because hydrogen peroxide solutions are likely to undergo changes while standing with respect to the amount of available oxygen, as is well understood in this art.

Various combinations of oxidizing agents may be used, but experiments have indicated that the most advantageous treatment involves the use of a mixture of sodium chloride and sodium chlorate in equal proportions, taking into account all of the factors which should be considered in evaluating complete effectiveness, including, in addition to those previously mentioned, over-all total cost and ease of operation. It will immediately be apparent that both of these oxidizing agents are quite inexpensive from the standpoint of re-agent costs, and that they are freely soluble under the conditions of operation. Moreover, they not only eliminate pitting but also effect substantial reduction in weight loss due to acid attack upon the basis metal while at the same time producing a completely acceptable degree of surface luster in only sixty seconds of treating time.

It will be at once apparent that the number of permutations possible in utilizing the process of the present invention is very large. But the essential principle involved is nonetheless quite simple. For this reason, the data supplied above is intended to show how the effectiveness of any given permutation can be quickly and accurately determined, and is not intended to demonstrate the full range of possibilities.

The oxidizing materials previously referred to (viz., the chlorides of sodium, potassium and ammonium, the chlorates and hypochlorites of sodium and potassium, and peroxide of hydrogen) have all proved to be effective in attaining one or more of the desired objectives, namely, reduction in weight loss, increase in luster or sheen, and substantial reduction of pitting.

In conclusion, it may be said that a most striking aspect of the present invention is the concept of adding an oxidizing material of the type described to a pickling bath for the treatment of titanium or zirconium, which bath contains a mixture of nitric and hydrofluoric acids. This is regarded as a most implausible proposal, for the scale and film to be removed are themselves the products of surface oxidation. It is surprising indeed that the result of this procedure is not only to produce a clean and brilliant surface, free of oxide film and scale, but also to reduce the weight loss due to acid attack upon the basis metal, and to practically eliminate pitting of the surface.

I claim:

1. In the process of finishing the surfaces of articles composed at least in major part of a metal selected from the class which consists of titanium and zirconium by treating such surfaces with an acid pickling bath to re move oxide film and scale, the improvement which consists in adding to an aqueous pickling bath containing an acidic agent which consists of hydrofluoric acid in an amount equal to from 1% to 6% by weight of the solution mixed with nitric acid in an amount equal to from 10% to 50% by weight of the solution, a minor amount ranging from 0.15% to 5% 'by weight of the solution of an agent selected from the class which consists of the chlorides of sodium, potassium and ammonium; the chlorates and hypochlorites of sodium and potassium; peroxide of hydrogen; and mixtures of the same, and subjecting the article to be treated to contact with the liquid so prepared under time and temperature conditions sufficient to remove said scale and film.

2. A process as defined in claim 1, characterized in that said agent is hydrogen peroxide, which is present in an amount at least as great as 0.15 grams per 100 ml. of the solution. 3. A process as defined in claim 1, characterized in that the said agent consists of at least one chloride-containing salt of a metal selected from the class which comprises sodium and potassium.

4. A process as defined in claim 3, in which the said agent consists of a mixture of sodium'chloride and sodium chlorate, and is present in an amount at least as great as 2 grams per liter in the aqueous solution.

5. A process as defined in claim 3, in Whichthe said agent consists of a mixture of sodium chloride and sodium chlorate in substantially equal proportions, and in which the mixture constitutes approximately 1.5 by weight of the aqueous acid solution.

6. The process of claim 1, in which the surface treated consists essentially of titanium metal, and the said agent constitutes a mixture of sodium chloride and sodium chlorate, representing approximately 1.5% by weight of the aqueous acid bath.

References Cited in the file of this patent UNITED STATES PATENTS 2,154,455 Kepfer Apr. 18, 1939 2,154,469 Oplinger Apr. 18, 1939 2,283,170 Batcheller May 19, 1942 2,542,727 Theuerer Feb. 20, 1951 2,577,803 Pfann Dec. 11, 1951 2,606,960 Little Aug. 12, 1952 2,711,364 Beach June 21, 1955. 2,740,699 Koury Apr. 3, 1956 

1.IN THE PROCESS OF FINISHING THE SURFACE OF ARTICLES COMPOSED AT LEAST IN MAJOR PART OF A METAL SELECTED FROM THE CLASS WHICH CONSISTS OF TITANIU AND ZIRCONIUM BY TREATED SUCH SURFACES WITH AN ACID PICKLING BATH TO REMOVE OXIDE FILM AND SCALE, THE IMPRVEMENT WHICH CONSISTS IN ADDING TO AN AQUEOUS PICKLING BATH CONTAINING AN ACIDIC AGENT WHICH CONSISTS OF HYFROFLUORIC ACID IN AN AMOUNT EQUAL TO FORM 1% TO 6% BY WEIGHT OF SOLUTION MIXED WITH NITRIC ACID IN AN AMOUNT EQUAL TO FROM 10% TO 50% BY WEIGHT OF THE SOLUTION, A MINOR AMOUNT RANGING FROM 0.5% TO 5% BY WEIGHT OF THE SOLUTION OF AN AGENT SELECTED FROM THE CLASS WHICH CONSISTS OF THE CHLORIDES OF SODIUM, POTASSIUM AND AMMONIUM; THE CHLORATES AND HYPOCHLORITES OF SODIUM AND POTASSIUM; PEROXIDE OF HYDROGEN; AND MIXTURES OF THE SAME, AND SUBJECTING THE ARTICLE TO BE TREATED TO CONTACT WITH THE LIQUID SO PREPARED UNDER TIME AND TEMPERATURE CONDITIONS SUFFICIENT TO REMOVE SSID SCALE AND FILM. 